The objectives of the present investigation are: (1) To continue the development and testing of a molecular model regarding the interactions of anti-arrhythmic drugs with cardiac sodium channels. (2) To characterize the structure-activity relationship for a series of lidocaine-procainamide derivatives. (3) To develop a single cardiocyte voltage clamp technique for studying the effects of antiarrythmic drugs upon the sodium current and its underlying mechanisms (e.g. gating currents) directly. The antiarrhythmic drug effects will be estimated either from their effects upon the maximum upstroke velocity of the action potential recorded from a standard single sucrose gap papillary muscle; or by measuring the peak sodium current of a single isolated cardiocyte, using the two microelectrode voltage clamp technique. It is hoped that this study will (1) Enhance our understanding of the mechanism of action of antiarrythmic drugs. (2) Elucidate the important biophysical properties of these molecules and thereby promote development of effective and safe new antiarrythmic drugs.